This invention relates to methods and arrangements for monitoring and controlling desulfurization of NOx reservoir catalysts in the exhaust of Otto or Diesel type internal combustion engines for exhaust gas purification.
NOx reservoir catalysts are poisoned during operation by sulfur contained in the fuel and incorporation of the sulfur as sulfate in the NOx reservoir catalyst, so that the NOx adsorption by the reservoir catalysts is impeded or prevented. Consequently a desulfurization of the NOx reservoir catalyst must be carried out at periodic intervals. It is known that sulfur poisoning of the NOx reservoir catalysts is largely or completely reversible, if the catalyst is subjected to a temperature above desulfurization temperature in a reducing environment. As used herein, the term "desulfurization" refers to the removal of sulfur in any form from a catalyst.
In the case of normally lean-operated Otto engines, it is possible to operate with a stoichiometric or a rich fuel-air mixture in any operating mode, at the expense of a corresponding increase in fuel consumption. By such measures as late ignition of the engine, moreover, the exhaust gas temperature is variable within wide limits, so that conventional desulfurization temperatures of about 650.degree. can be achieved in practically any portion of the operating diagram of an Otto engine.
NOx reservoir catalysts can not incorporate any NOx under lean fuel conditions at temperatures above about 550.degree., and they also exhibit marked ageing effects when a temperature of about 800.degree. is exceeded.
In the case of Diesel engines, useful operation at .lambda..ltoreq.1 is not feasible because of the resulting power loss and increased particulate emission.